[FIX] mass_mailing: unsubscribe link when testing the mailing
[FIX] mass_mailing: unsubscribe link when testing the mailing

When testing a mass mailing before sending the mailing
to the contacts (which well, is always the case),
the local links were not replaced by absolute links,
and the override done on the method `send_get_email_dict`
to convert the link `/unsubscribe_from_list` to the actual
unsubscription link was not working, as it relied
on the absolute link.

opw-657982

laslabs/odoo,ygol/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,bplancher/odoo,stephen144/odoo,dfang/odoo,storm-computers/odoo,sysadminmatmoz/OCB,optima-ict/odoo,microcom/odoo,stephen144/odoo,microcom/odoo,dfang/odoo,stephen144/odoo,sysadminmatmoz/OCB,storm-computers/odoo,storm-computers/odoo,dfang/odoo,hip-odoo/odoo,optima-ict/odoo,storm-computers/odoo,ygol/odoo,ygol/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,microcom/odoo,laslabs/odoo,microcom/odoo,storm-computers/odoo,hip-odoo/odoo,ygol/odoo,storm-computers/odoo,stephen144/odoo,bplancher/odoo,microcom/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,dfang/odoo,sysadminmatmoz/OCB,optima-ict/odoo,laslabs/odoo,hip-odoo/odoo,sysadminmatmoz/OCB,bplancher/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,stephen144/odoo,ygol/odoo,sysadminmatmoz/OCB,microcom/odoo,ygol/odoo,optima-ict/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,optima-ict/odoo,dfang/odoo,dfang/odoo,stephen144/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,laslabs/odoo,laslabs/odoo,sysadminmatmoz/OCB,optima-ict/odoo,bplancher/odoo,sysadminmatmoz/OCB